Field of the Disclosure
The present disclosure relates to an image processing apparatus and image processing method for processing an image of a subject.
Description of the Related Art
Optical coherence tomography (hereinafter referred to OCT) has been used as a non-destructive and non-invasive method for obtaining a tomographic image of a measurement target, such as a living body. OCT is widely used especially in the field of ophthalmology to obtain tomographic images of a retina in a fundus of an eye to be examined for ophthalmic examination of the retina and the like.
In OCT, light reflected from a measurement target interferes with light reflected from a reference mirror, and the time- or wavenumber-dependence of the intensity of the interfering light is analyzed to obtain tomographic images. Examples of a known apparatus for obtaining such an OCT image include a time domain OCT, a spectral domain OCT (SD-OCT), and a swept source OCT (SS-OCT). The time domain OCT obtains depth information about a measurement target by moving a reference mirror to different positions. The SD-OCT uses a broad-bandwidth light source. The SS-OCT uses as a light source a wavelength-tunable light source in which an oscillation wavelength is tunable. The SD-OCT and the SS-OCT are collectively referred to as Fourier domain OCT (FD-OCT).
In recent years, a virtual angiography using FD-OCT has been discussed, and such virtual angiography is called OCT angiography (OCTA). A fluorescein angiography, which is a commonly-used angiography in modern clinical practice, requires injection of a fluorescent dye (e.g., fluorescein or indocyanine green) into a body and produces a two-dimensional representation of blood vessels through which the fluorescent dye passes. On the other hand, the OCTA enables a non-invasive virtual angiography and can provide a three-dimensional representation of a network of blood flow. Furthermore, the OCTA has a higher resolution than the fluorescein angiography and can produce images of fine blood vessels or blood flow in the eye fundus. For this reason, the OCTA has been attracting rising attention.
A method of detecting blood flow with the OCTA is discussed in Fingler et al. “Mobility and transverse flow visualization using phase variance contrast with spectral domain optical coherence tomography”, Optics Express, Vol. 15, No. 20, pp 12637-12653 (2007). In the method discussed therein, only time-modulated signals are extracted from OCT signals to separate the OCT signals that are from blood flow. Other methods for detecting blood flow are also discussed including a method which uses intensity fluctuations due to blood flow (United States Patent Application Publication No. 2014/221827) and a method which uses phase fluctuations due to blood flow. As used herein, an image showing time-modulated signals which are signals among OCT signals is sometimes referred to as a “motion contrast image”, a pixel value of the motion contrast image as “motion contrast”, and a data set of the motion contrast as “motion contrast data”.
Meanwhile, a polarization OCT developed as a functional OCT is capable of visualizing structural information, such as a nerve fiber layer and a retinal layer. Investigative Ophthalmology & Visual Science, Jan. 7, 2013, Zotter S et al., “Measuring retinal nerve fiber layer birefringence, retardation, and thickness using wide-field, high-speed polarization sensitive spectral domain OCT” discusses a technique for obtaining en face maps using the polarization OCT by integrating, along a thickness direction, three-dimensional data per unit thickness on a retinal nerve fiber layer (RNFL) deflection parameter called retardation.